Fuel cell system

ABSTRACT

The present application relates to a fuel cell system comprising a fuel cell stack ( 1 ) which has layers of a plurality of fuel cells ( 2 ) which are each separated from one another by bipolar plates ( 3; 3 ′). The bipolar plates have openings for cooling ( 4 ) or for the supply ( 5   a ) and discharge ( 5   b ) of media to/from the fuel cells. The fuel cell stack can be placed under mechanical compressive stress in the direction ( 6 ) of the layering. Resilient bead arrangements ( 7; 7 ′) are provided at least in regions to seal the openings ( 4, 5   a   , 5   b   , 10 ).

The present invention relates to a fuel cell system in accordance withthe preamble of claim 1.

Fuel cell systems are known in which a stack of fuel cells isconstructed with layers of a plurality of fuel cells which are eachseparated from one another by bipolar plates. The bipolar plates herehave a plurality of functions:

-   -   electrical contacting of the electrodes of the fuel cells and        passing the current on to the adjacent cell (serial connection        of the cells),    -   supplying the cells with reaction gases and e.g. to drain the        produced reaction water via a corresponding channel structure,    -   passing on the heat produced during the reaction in the fuel        cell, and    -   sealing of the various gas or cooling channels from one another        and towards the outside.

For supplying media to and draining media from the bipolar plates to theactual fuel cells (these are for example MEAs (Membrane ElectronAssembly), each having a gas-diffusion layer formed for example from acarbon mat and orientated towards the bipolar plates), the bipolarplates have openings for cooling or for supplying and draining media.

In particular with respect to the gas-diffusion layer, difficultiesregularly arise here. Up to now it has been usual to design the sealingbetween the bipolar plates or respectively between the bipolar platesand the fuel cell in that for example an elastomer seal is inserted intoa groove of the bipolar plate. By exertion of compressive stress (forexample by means of tightening straps) on the fuel cell stack,compression of the seal occurs by which means a sealing effect for theopenings is intended to be achieved.

Now a problem with the inserted gas-diffusion layer is that it isgenerally designed as a graphite fibre mat or graphite paper. Standardindustrial graphite fibre mats have a nominal thickness of e.g. 340 μm;however the production tolerance is around ±40 μm. The graphite fibreswhich form the mat are themselves brittle and not resilient. Moreover itis also not recommended here to balance production tolerances of thegraphite fibre mat by compressing the mat since the gas permeability ofthe gas diffusion layer is severely impaired by this and thus theoperation of the fuel cell is restricted. On the other hand it isnecessary, however, to exert a minimum pressure through the bipolarplate on the entire gas-diffusion layer so that there is adequateelectrical conductivity through the gas-diffusion layer.

It can therefore be summarised that with the previous elastomer sealseither an imperfect sealing effect or a non-optimal operation of thefuel cell had to be accepted. Added to this, especially in the case offuel cells operated with molecular hydrogen, losses of H₂ occur due tothe gas diffusing through the elastomer seal.

The object underlying the present invention, therefore, is to achievereliable sealing of the openings in a fuel cell stack with the lowestpossible costs.

This object is accomplished by a fuel cell system according to claim 1.

Because resilient bead arrangements are provided at least in regions toseal the openings, reliable sealing is achieved over a long resilientpath (a wide range of elastic compressibility) of the bead arrangement.“Openings” are understood in the present application as a region ofpractically any type which is to be sealed. This can be for example aport for a reaction gas or a coolant. It can however also be e.g. theelectrochemically active region in which e.g. the gas-diffusion layer isarranged or screw holes are provided. The resilient bead arrangementconstantly permits the balancing of production tolerances of e.g.gas-diffusion layers within a wide tolerance range and yet the provisionnevertheless of an optimal sealing effect.

Advantageous embodiments of the invention are described in the dependentclaims.

A very advantageous embodiment of the invention provides for the beadarrangement of thin coating for the micro-sealing having a thickness ofbetween 1 μm and 400 μm. The coating is advantageously formed from anelastomer such as silicon, viton or EPDM; these are applied for examplein a screen printing process or by CIPG (cure-in-place gasketing; i.e.elastomer introduced in a fluid state at the location of the seal andcured there). What is achieved by these measures is that, for example,the hydrogen diffusion through the seal is reduced to a very low amount.

A further advantageous embodiment of the invention provides for the beadarrangement to contain a complete bead or a half bead. Here it is alsopossible to provide both forms within one bead arrangement, sinceaccording to the course of the bead arrangement in the plane, otherelasticities can prove to be sensible, e.g. in narrow radii a differentbead geometry could be more sensible compared to straight courses of thebead arrangement.

A further advantageous embodiment provides for the bead arrangement tobe formed from steel. Steel offers the advantage that its machining ispossible with conventional tools in a very cost-effective way, moreovere.g. methods for coating steel with thin elastomer layers are welltried. The good elastic properties of steel make it possible to formwell the wide range of elastic compressibility offered by the invention.Here it is particularly favourable for the bead arrangement to beattached to the bipolar plate. This gives on the one hand thepossibility that the bipolar plate is designed overall as a formed steelpart (which is perhaps provided in regions with a coating to resistcorrosion). However it is also possible that the bipolar plate isdesigned as a composite element of two steel plates with a plasticsmaterial plate lying between them. In each case the good productionpossibilities of steel can be exploited; it is possible to provide thebead arrangement within a process step which is taking place anyhow(e.g. the imprinting of a flow field). Thus very low costs arise; thereare also no additional error sources due to extra components, such asadditionally inserted elastomer seals.

However, it is also possible according to the invention to provide thebead arrangement from other metals, such as steel, nickel, titanium oraluminium for example. The choice of preferred metal here depends alsoon the desired electrical properties or on the desired degree ofcorrosion resistance.

Thus it is possible to adapt the compression characteristic of the beade.g. to a gas-diffusion layer. This does not however have to apply onlyto gas-diffusion layers; the bead characteristics can in general be welladapted to components which have less elasticity. The beaded seal can bedesigned flexible and thus can be well used by all fuel cellmanufacturers without high re-equipping costs.

A further advantageous embodiment provides for the bead arrangement tohave a stopper which limits the compression of the gas-diffusion layerto a minimum thickness. This is an incompressible part of the beadarrangement or respectively a part, the elasticity of which is muchlower than that of the actual bead. By this means the degree ofdeformation in the bead region is limited so that the bead cannot bepressed completely flat.

A further advantageous embodiment provides for the bead arrangement tobe disposed on a separate component from the bipolar plate. This isparticularly propitious when the bipolar plates consist of a materialsuch as graphite which is unsuitable for bead arrangements. The separatecomponent is then placed on the bipolar plate or integrated by gluing,clicking in, welding, soldering in, or by injection mold-around, so thataltogether a sealing connection is provided between the separatecomponent and the bipolar plate.

Finally a further advantageous embodiment provides for the beadarrangement to be realised from an elastomer bead. Such a bead can beapplied in a screen printing process. This serves both micro- andmacro-sealing. The bead also assumes the function of adapting thecompression to a gas-diffusion layer.

Further advantageous embodiments of the present invention are given inthe remaining dependent claims.

The present invention is now explained with the aid of several figures.These show:

FIGS. 1 a to 1 c the type of setup of a fuel cell stack,

FIGS. 2 a + 2 b embodiments of bead arrangements according to theinvention,

FIG. 2 c a plan view of a bipolar plate according to the invention,

FIGS. 3 a to 3 d several bead arrangements with stoppers.

FIG. 1 a shows the setup of a fuel cell arrangement 12 such as is shownin FIG. 1 b. A plurality of fuel cell arrangements 12 forms, in layers,the region of a fuel cell stack 1 disposed between end plates (see FIG.1 c).

In FIG. 1 a can be seen a fuel cell 2 with its regular components, whichhas an ion-conductive polymer membrane which is provided on both sidesin the central region 2 a with a catalyst layer. In the fuel cellarrangement 12 are provided two bipolar plates 3 between which the fuelcell 2 is disposed. In the region between each bipolar plate and thefuel cell is moreover arranged a gas-diffusion layer 9 which is of suchdimensions that it can be accommodated in a recess in the bipolar plate.In the assembled state of the fuel cell 12, the electrochemically activeregion of the fuel cells, which is substantially covered by thegas-diffusion layer, is arranged in a substantially closed space 10(this corresponds substantially to the above-mentioned recess in thebipolar plate), which is substantially circumferential limited by a bead11. This closed space 10 is gas-tight due to the bead 11 which is partof a bead arrangement 7 or 7′ (see FIGS. 2 a and 2 b).

Ports for the drain of media 5 a and for the discharge of media 5 b liewithin the sealing region and are sealed by the bead 11 from additionalports, for instance the ports for cooling 4 (which have their own beadfor sealing). The sealing effect here takes place on all the beads byexertion of compression force on the fuel cell stack 1 in the direction6 of the layering (see FIG. 1 c). This takes place e.g. by means oftightening bands which are not shown here. The bead 11 offers theadvantage that it has a wide range of elastic compressibility in whichit shows an adequate sealing effect. This is particularly advantageouswith the incorporation of the gas-diffusion layer 9, formed from agraphite fibre mat, which is produced in the industry with highproduction tolerances. Due to the wide range of elastic compressibilityof the bead 11, adaptation of the bead to the geometry of thegas-diffusion layer is possible. What is thereby achieved is that on theone hand lateral sealing is achieved and on the other hand both anadequate gas distribution in the gas-diffusion layer plane is given andmoreover the pressure distribution in the direction of layering 6 isuniform and sufficiently high to achieve uniform electric conductivitythrough the gas-diffusion layer. To improve the micro-sealing, the bead11 is provided on its outer side with a coating of an elastomer whichhas been applied in a screen printing process.

In order to limit the compression of the gas-diffusion layer, the beadconstruction is designed with a stopper. This stopper, which can bedesigned as a crimped portion, a a wave stopper or also as a trapezoidstopper, will be described in more detail below in the description ofFIGS. 3 a to 3 d. All the stoppers have the function that they can limitthe compression of the bead to a minimum height.

The bipolar plate 3 is here designed as a metal part. In respect of theease of manufacture and the advantages of steel in connection with beadarrangements, reference is made to what has already been said.

If the bipolar plate is formed e.g. from a metal which is not suitablefor the production of appropriate bead geometries having the necessaryelasticity, the bead region can be formed from some other suitablematerial (e.g. steel). By joining processes such as welding, soldering,gluing, riveting, clicking in, connection of the separate bead componentto the bipolar plate then takes place. If the bipolar plates are made ofa material other than metal, for example from graphite, graphitecomposite or plastics material, the bead region can be designed as aframe formed from a suitable material. By joining methods such asmelting, injection mold-around, welding, soldering, gluing, riveting,clicking in, the base material of the bipolar plate, which contains theflow field, is connected to a bead sealing frame, which contains thebeads, in a gas- or fluid-tight manner.

FIGS. 2 a and 2 b show two embodiments of a bead arrangement accordingto the invention. In FIG. 2 a is shown a cross-section through the beadarrangement 7 which shows the bead 11, configured as a half bead. Thesubstantially circumferential bead 11 encloses, as already explained inthe remarks relating to FIG. 1 a, the gas-diffusion layer 9. In FIG. 2a, the bead 11 is designed as a so-called half bead, i.e. for example inthe shape of quarter of a circle. Since the inner region of the fuelcell has to be enclosed by a seal, and there are intersections in theregion of the media channels (see FIG. 2 c), an alternatingconfiguration as a complete bead and as a half bead is necessary. A fullbead can here merge into two half beads, which then each have their ownsealing effect.

In addition, the use of a full bead or respectively a half bead offersthe possibility of adapting the elasticity within wide limits.

FIG. 2 a shows the bead arrangement 7 in the uncompressed state. Whenmechanical compressive stress is exerted on the fuel cell stack,compression takes place in direction 6, such that the bead arrangement 7or respectively the bead 11 forms a gas-tight lateral seal for theclosed space 10 in respect of the gas-diffusion layer.

FIG. 2 b shows a further bead arrangement, the bead arrangement 7′. Theonly difference between this arrangement and that of FIG. 2 a consistsin the fact that here a bead 11′ is formed as a full bead (here almostwith a semi-circular cross-section). There are numerous otherembodiments of the present invention. Thus it is for example possible toshow other bead geometries than those shown here; multiple beads arealso possible. Moreover the bead seal according to the invention ispossible for all the seals in the region of the fuel cell stack to becompressed. Thus it is not only possible to seal the electrochemicallyactive region around the gas-diffusion layer, but also any passages forgaseous or fluid media etc. In sealing around the fuel cells stackassembly guide (screw holes), the elasticity of a bead arrangement canbe used to counteract displacement in the stack and compensate forpossible tolerances.

FIG. 2 c shows a plan view of an additional embodiment 3′ of a bipolarplate according to the invention. Here the bead arrangements can berecognised in the plan view by a broad line. The bead arrangements hereserve to seal a plurality of ports.

FIGS. 3 a to 3 d show various bead arrangements which each have astopper. This stopper serves to limit the deformation of a bead in sucha way that it cannot be compressed below a certain specific height.

Thus FIG. 3 a shows a single-layer bead arrangement which has a fullbead 11″, the deformation of which is limited in direction 15 by acorrugated stopper 13. FIG. 3 b shows a two-layer bead arrangement inwhich a full bead of the upper layer is limited in deformation by afolded metal sheet lying underneath it. FIGS. 3 c and 3 d show beadarrangements in which at least two complete beads face one another andeither a crimped metal sheet (see FIG. 3 c) or a corrugated metal sheet(see FIG. 3 d) is provided to limit deformation.

1-14. (cancelled)
 15. A fuel cell system comprising: a plurality of fuelcells having an electrochemically active region; a plurality of bipolarplates separating said fuel cells, said bipolar plates having at leastone opening and at least one resilient bead; wherein said at least oneopening transports coolant/media through the fuel cell system, and saidat least one resilient bead sealingly encloses said at least one openingover a wide range of elastic compressibility; and wherein said fuel cellsystem is under compressive stress in the direction of the fuel cells.16. The fuel cell system according to claim 15, wherein said at leastone resilient bead is integrated into said bipolar plate.
 17. The fuelcell system according to claim 15, wherein said at least one resilientbead additionally sealingly encloses said electrochemically activeregion of said fuel cell.
 18. The fuel cell system according to claim 15additionally comprising a gas diffusion layer disposed between said fuelcell and said bipolar plate.
 19. The fuel cell system according to claim18, wherein said gas diffusion layer is constructed of a materialselected from the group consisting of conductive fabric, graphite mat,and graphite paper.
 20. The fuel cell system according to claim 15,wherein said at least one resilient bead additionally comprises acoating for improved sealing.
 21. The fuel cell system according toclaim 20, wherein said coating is an elastomer.
 22. The fuel cell systemaccording to claim 20, wherein said coating is deposited by a methodselected from the group consisting of screen printing, tampon printing,and cure-in-place gasketing.
 23. The fuel cell system according to claim15, wherein said at least one resilient bead is a full bead.
 24. Thefuel cell system according to claim 15, wherein said at least oneresilient bead is a half bead.
 25. The fuel cell system according toclaim 15, wherein said at least one resilient bead is constructed of amaterial selected from the group consisting of steel, nickel, titanium,and aluminum.
 26. The fuel cell system according to claim 15, whereinsaid bipolar plate is a formed metal part.
 27. The fuel cell systemaccording to claim 18, wherein said at least one resilient beadadditionally comprises a stopper, said stopper limiting compression ofsaid gas diffusion layer.
 28. The fuel cell system according to claim17, wherein said at least one bipolar plate additionally comprises: afirst plate; a second plate; and a separator disposed between said firstplate and said second plate.
 29. The fuel cell system according to claim28, wherein said first plate and said second plate are metal.
 30. Thefuel cell system according to claim 28, wherein said separator isplastic.
 31. A fuel cell system comprising: a plurality of fuel cellshaving an electrochemically active region; a plurality of bipolar platesseparating said fuel cells, said bipolar plates having at least oneopening transporting coolant/media through the fuel cell system; a beadcarrier having a at least one resilient bead, said bead carrier disposedbetween said fuel cell and said bipolar plate; wherein said at least oneresilient bead sealingly encloses said at least one opening over a widerange of elastic compressibility; and wherein said fuel cell system isunder compressive stress in the direction of the fuel cell layering. 32.The fuel cell system according to claim 31, wherein said bipolar plateis constructed of a material selected from the group consisting ofgraphite, plastic, and metal.
 33. The fuel cell system according toclaim 31, wherein said bead carrier is fixingly attached to said bipolarplate by a method selected from the group consisting of gluing, snap-in,welding, soldering and injection mold-around.